MIDWEST STATES POOLED FUND PROGRAM

Phase II Conceptual Development of an Impact Attenuation System for Intersecting Roadways

Sponsoring Agency Code

TPF-5(193) Supplement 86

Problem Statement

Background

Treatments for intersecting roadways are intended to protect motorists from the difficult
problem created when an intersection is placed near a bridge railing. Currently, there are
no safety treatments for this situation that have been successfully tested for high-speed
applications. A design that can safely treat this situation along high-speed roadways is
sorely needed. In addition, the new design concept must focus on the site and space
restraints associated with intersecting roadways and adapt a design that best meets those
constraints.

Previously, the Midwest States Pooled Fund Program funded a series of projects with the
Midwest Roadside Safety Facility (MwRSF) to develop a new short radius guardrail system
for use at intersecting roadways that are in close proximity to a bridge or other structures.
This R&D effort proved to be a very daunting task. However, it should be noted that the
prototype short radius guardrail system showed that significant improvement was achieved
when compared to the existing NCHRP Report No. 230-compliant barrier system.
Unfortunately, the system length and backside space requirements made the system
impractical to use in most real-world applications. In addition, any improvements deemed
necessary to help the system meet current impact safety standards would only further
increase system length. As such, there exists a need to develop a new barrier system that
can be used near intersecting roadways, driveways, and streets that would meet the TL-3
guidelines provided in MASH.

The Nebraska Department of Roads (NDOR) funded the first phase of this effort (M332 â€“
New Conceptual Development of an Impact Attenuation System for Intersecting
Roadways). This Phase I effort consisted of development of design concepts, analysis of
those concepts, and recommendations as to their feasibility. The project was proposed as
an initial conceptual design effort, allowing NDOR to limit the research funds for this phase
until a viable design was identified and a more substantial investment could be made
toward compliance testing.

The Phase I study focused on the design and analysis of various concepts for treatment of
intersecting roadways. The design concepts considered shielding a relatively wide area,
including both the bridge end near the intersection as well as the hazard posed by the
waterway and associated slopes within the clear zone. A number of initial design concepts
were considered, but the researchers and project sponsors focused the study on three
main design concepts:

1. A hybrid end terminal/crash cushion and net attenuator system that used the
terminal/crash cushion to shield the majority of the impacts along the primary
roadway, while the net attenuator shielded impacts farther down the secondary
roadway.
2. A hybrid end terminal/crash cushion and sand barrel attenuator system that used the
terminal/crash cushion to shield the majority of the impacts along the primary
roadway, while the sand barrel attenuator shielded impacts farther down the
secondary roadway.
3. A hybrid bullnose system that used increased width to redirect and/or capture
vehicles and a secondary energy-absorbing system inside the bullnose to shorten its
effective vehicle capture length.

A combination of engineering analysis, computer simulation, and dynamic component
testing were used to evaluate the three design concepts. All three systems demonstrated
potential for use in the treatment of intersecting roadways. However, the hybrid end
terminal/crash cushion and sand barrel attenuator and the hybrid bullnose system both
posed greater operational and constructability concerns due to their complexity and
relatively large footprint. Thus, the hybrid end terminal/crash cushion and net attenuator
was selected for further study based on its potential safety performance, its relatively clean
design and ease of maintenance, and the potential to accommodate moderate slopes
behind the system.

Following the Phase I study, the hybrid end terminal/crash cushion and net attenuator
system had several areas in need of further development. First, dynamic component testing
of the proposed Dragnet attenuator found that the current force levels were insufficient to
maintain stopping distances near the desired length of 30 ft. In fact, component testing with
three standard Dragnet energy absorbers on each side of the system resulted in deflections
over 40 ft. Thus, redesign of the net attenuator system will be required to increase the
resistive force and shorten the stopping distances. This will likely require redesign of the
energy-absorbing drums, the capture net, and the anchorage of the energy absorbers.
Additionally, it was desired that the hybrid end terminal/crash cushion and net attenuator
attempt to accommodate moderate slopes. Thus, additional research is needed to
determine what slopes can be safely used with the revised net attenuator. The first phase
of the research considered a variety of end terminal and crash cushion systems, but
additional research is needed to determine what other systems are optimal based on their
geometry and shielding of the bridge rail end. Finally, additional research is needed to
determine the exact layout of the hybrid end terminal/crash cushion and net attenuator
system in order to ensure that the two systems function properly when used together.

Thus, the current research results indicated a potential for an alternative design to meet the
MASH safety criteria. However, further research is needed to complete the design and
prepare it for full-scale crash testing and evaluation to MASH TL-3.

Objectives

The objective of this study is to pursue the long term development of a MASH-compliant
attenuation system for intersecting roadways utilizing a minimal footprint. Phase II,
proposed herein, would consist of the continued development of the preferred hybrid
terminal/crash cushion and net attenuator concept identified in Phase I. Specifically, the
Phase II research will address the design and performance evaluation of a net attenuation
system that utilizes increased force levels to shorten the vehicle stopping distance,
evaluation of the revised net attenuator on slopes, design of the revised net attenuation
system anchorage, design of the layout of the new net attenuation system to interact safely
with existing terminal and crash cushion systems, development of design details for
fabrication of the hybrid system, and determination of the test matrix required for full-scale
crash testing and evaluation to MASH TL-3. Full-scale compliance testing of any proposed
system would require additional funding.

Research Plan

MwRSF will work closely with NDOR engineers and the TAC committee members
throughout the concept development of a new attenuation system for intersecting roadways
in order to ensure that the system is practical. This focus should ensure that the system is
viable for NDOR as well as other state DOTâ€™s.

Once the new, TL-3 attenuation system for intersecting roadways has been crash tested,
evaluated, and accepted by FHWA, NDOR and other State DOTs can implement the new
design into its Standards and/or Special Plans for intersecting roadways. At the conclusion
of this research project, it is recommended that NDOR designate an intersecting roadway
location that will use this new technology in order to evaluate a â€œreal-worldâ€ installation and
make any necessary improvements.

Finally, the publication and dissemination of the research results and demonstration
program, in the form of newsletters, research reports, and refereed journal papers, will aid
the rapid transfer of this new technology to all interested organizations.

The research effort for this study will focus on the continued development of a hybrid end
terminal/crash cushion and net attenuator, including development and evaluation of highperformance
net attenuation system, design of the integration of the net attenuation system
with a selected crash cushion/end terminal design, determination of the appropriate test
matrix for full-scale crash testing, and recommendations for future work. The details of
these tasks are outlined below in two sections, design and evaluation of high-performance
net attenuation system and design of integrated terminal/crash cushion and net attenuator.

Development of High-Performance Net Attenuation System

As noted previously, initial dynamic component testing and analysis of a modified Dragnet
system found that the system had the potential safely decelerate vehicles in much shorter
distances than the standard system. However, the force levels in the system were not
sufficient to bring vehicle stopping distances into the desired 30 ft range. MwRSF has
contacted Impact Absorption Inc., makers of the Dragnet system. They have indicated a
willingness to collaborate with MwRSF and develop a revised energy absorber and
increased capacity net attenuator. Thus, MwRSF will work with Impact Absorption Inc. to
develop a revised energy absorber that will increase the force level of the absorbers to
18,000 lbs or more. Additionally, the size and footprint of the absorbers, the connection to
the capture net, the capacity of the capture net, and anchorage of the attenuator will be
considered in the design effort. Potential designs will be reviewed with NDOR.

Once the design of a high-performance net attenuation system is achieved, dynamic
component testing will be used to evaluate the safety performance of the new design as
well as evaluate its potential use on moderate slopes. Seven dynamic component tests are
proposed to evaluate the new design. First, three high-speed bogie tests will be conducted
with MwRSFâ€™s 5,000-lb bogie vehicle to evaluate stopping distances and stability under
various impact orientations. It is proposed that the high-performance net attenuation
system be evaluated with a perpendicular impact in the center of the system, a
perpendicular impact offset to one side of the net, and an angled impact offset to one side
of the net. Next, two high-speed bogie tests will be conducted with MwRSFâ€™s 1,800-lb bogie
vehicle to evaluate potential occupant risk criteria and vehicle stability concerns for small
passenger vehicles. These tests would include a perpendicular impact in the center of the
system and an angled impact offset to one side of the net. Finally, two high-speed bogie
tests will be conducted with MwRSFâ€™s 5,000-lb bogie vehicle to evaluate the potential for
the high-performance net attenuation system to be used on slopes. Thus, testing will be
conducted on 6:1 and 8:1 slopes. The slope configurations may have both longitudinal and
lateral slopes depending on input from NDOR.

Design of Integrated Hybrid Terminal/Crash Cushion and Net Attenuator

The second component of the research effort will integrate the high-performance net
attenuation system with an end terminal/crash cushion. The first step in that process will be
review of existing end terminal and crash cushion systems to determine those systems
have the best potential for use in the hybrid design. The review will examine factors such
as working width, length, stopping distance, transition to the bridge rail, etc. This review will
then be used to select an end terminal/crash cushion system for use in the continued
development and evaluation of the system. Again, review and comment from NDOR will be
incorporated into the selection process.

Once an end terminal/crash cushion system has been selected, the research effort will shift
to design and evaluation of an anchorage for the high-performance net attenuation system.
It is anticipated that the high-performance net attenuation system will require increased
anchorage due to the higher loads it will develop. In addition, the anchorage must consider
integration with the end terminal/crash cushion, cost, and installation factors. Thus, MwRSF
will brainstorm a variety of anchorage concepts and present them to NDOR for review. The
concepts will consider anchorage that is entirely independent of the end terminal/crash
cushion as well as designs that connect to the end terminal/crash cushion in some manner.
Following review of the concepts, a preferred anchorage design will be chosen, and
structural design and CAD details of the anchorage will be developed. One dynamic
component test of the anchorage will be used to verify its capacity. If the desired capacity is
not met, the results from the test will be used to redesign the anchorage to meet the
targeted capacity.

The final task involves the integration of the high-performance net attenuation system with
an end terminal/crash cushion. This effort will include selection of the location of net
attenuator relative to the end terminal/crash cushion based on potential vehicle impacts
and orientations, including considerations for shielding of the clear zone and associated
hazards, removing gaps between the two systems, and optimizing the function of the two
systems without interference that may compromise safety. Once the design of the layout is
complete, CAD details of the proposed system design will be developed.

At the end of the research effort, a summary report will be completed that details the hybrid
end terminal/crash cushion and net attenuator research effort and provides
recommendations for future study. It is anticipated that future research could follow one of
two paths. If the research effort described herein results in a design for which researchers
have high-confidence, then full-scale crash testing under MASH TL-3 would be the logical
next step. As such, MwRSF will develop a crash test matrix for the proposed design as part
of the summary report. Alternatively, NDOR may want to pursue computer simulation
efforts to build further confidence in the design. MwRSF will provide recommendations in
the summary report for both alternatives. MwRSF will also prepare a technical brief and a
PowerPoint presentation of the research results to NDOR at the completion of the project.

Benefit

Currently, no safety treatment has been successfully crash tested using TL-3 conditions
under NCHRP Report No. 350 or MASH to resolve the problems posed when intersecting
roadways are located near a bridge railing. A design that can safely treat this situation
along high-speed roadways is sorely needed. In addition, the development of a new design
concept for an attenuation system for intersecting roadways will focus on the site and
space restraints associated with intersecting roadways and adapt a design that best meets
those constraints.

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